Plumes of trichloroethene (TCE) with degradation products occur at a large industrial site in California where TCE as a dense non-aqueous phase liquid (DNAPL) entered the fractured sandstone bedrock at many locations beginning in the late 1940s. Groundwater flows rapidly in closely spaced fractures but plume fronts are strongly retarded relative to groundwater flow velocities owing largely to matrix diffusion in early decades and degradation processes in later decades and going forward. Multiple data types show field evidence for both biotic and abiotic dechlorination of TCE and its degradation products, resulting in non-chlorinated compounds...

A previous study has reported that Knudsen diffusion coefficients obtained by tracer experiments conducted with a binary gas system and a porous medium are consistently smaller than those obtained by permeability experiments conducted with a single-gas system and a porous medium. To date, however, that study is the only one in which tracer experiments have been conducted with a binary gas system. Therefore, to confirm this difference in Knudsen diffusion coefficients, we used a method we had developed previously to conduct tracer experiments with a binary carbon dioxide-nitrogen gas system and five porous media with permeability coefficients ranging from 10-13 to 10-11 m2 ...

Non-pumping reactive wells (NPRWs) filled by zero-valent iron (ZVI) can be utilized for the remediation of groundwater contamination of deep aquifers. The efficiency of NPRWs mainly depends on the hydraulic contact time (HCT) of the pollutant with the reactive materials, the extent of the well capture zone (Wcz ), and the relative hydraulic conductivity of aquifer and reactive material (Kr ). We investigated nitrate removal from groundwater using NPRWs filled by ZVI (in nano and micro scales) and examined the effect of NPRWs orientations (i...

The effect of groundwater viscosity control on the performance of surfactant-enhanced air sparging (SEAS) was investigated using 1- and 2-dimensional (1-D and 2-D) bench-scale physical models. The viscosity of groundwater was controlled by a thickener, sodium carboxymethylcellulose (SCMC), while an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), was used to control the surface tension of groundwater. When resident DI water was displaced with a SCMC solution (500 mg/L), a SDBS solution (200 mg/L), and a solution with both SCMC (500 mg/L) and SDBS (200 mg/L), the air saturation for sand-packed columns achieved by air sparging increased by 9...

Recalcitrant organic contaminants, such as 1,4-dioxane, typically require advanced oxidation process (AOP) oxidants, such as ozone (O3 ), for their complete mineralization during water treatment. Unfortunately, the use of AOPs can be limited by these oxidants' relatively high reactivities and short half-lives. These drawbacks can be minimized by partial encapsulation of the oxidants within a cyclodextrin cavity to form inclusion complexes. We determined the inclusion complexes of O3 and three common co-contaminants (trichloroethene, 1,1,1-trichloroethane, and 1,4-dioxane) as guest compounds within hydroxypropyl-β-cyclodextrin...

Groundwater-quality assessment at contaminated sites often involves the use of short-screen (1.5 to 3 m) monitoring wells. However, even over these intervals considerable variation may occur in contaminant concentrations in groundwater adjacent to the well screen. This is especially true in heterogeneous dense non-aqueous phase liquid (DNAPL) source zones, where cm-scale contamination variability may call into question the effectiveness of monitoring wells to deliver representative data. The utility of monitoring wells in such settings is evaluated by reference to high-resolution multilevel sampler (MLS) wells located proximally to short-screen wells, together with sampling capture-zone modelling to explore controls upon well sample provenance and sensitivity to monitoring protocols...

Hydraulic fracturing (or fracking) is a well stimulation technique used to extract resources from a low permeability formation. Currently, the most common application of fracking is for the extraction of oil and gas from shale. During the operation, a large volume of brine, rich in hazardous chemicals, is produced. Spills of brine from wells or pits might negatively impact underground water resources and, in particular, one of the major concerns is the migration of radionuclides, such as radium (Ra2+), into the shallow subsurface...

Diffusion cell and diffusion wafer experiments were conducted to compare methods for estimating effective matrix diffusion coefficients in rock core samples from Pahute Mesa at the Nevada Nuclear Security Site (NNSS). A diffusion wafer method, in which a solute diffuses out of a rock matrix that is pre-saturated with water containing the solute, is presented as a simpler alternative to the traditional through-diffusion (diffusion cell) method. Both methods yielded estimates of effective matrix diffusion coefficients that were within the range of values previously reported for NNSS volcanic rocks...

Silver nanoparticles (Ag-NP) are used in various consumer products and are one of the most prevalent metallic nanoparticle in commodities and are released into the environment. Transport behavior of Ag-NP in groundwater is one important aspect for the assessment of environmental impact and protection of drinking water resources in particular. Ag-NP transport processes in saturated single-fractured sandstones using triaxial flow cell experiments with different kind of sandstones is investigated. Ag-NP concentration and size are analyzed using flow field-flow fractionation and coupled SEM-EDX analysis...

Groundwater denitrification bioreactors under limited phosphorus conditions were studied in column experiments using four types of plant-chips. When the phosphate-P concentration in the influent increased from 0.04mg/L to 0.4mg/L, the nitrate removal ratio increased from 61.6% to 86.1% in reed, from 7.2% to 12.6% in Japanese cedar, from 37.0% to 73.6% in Moso bamboo, and from 19.2% to 50.5% in Lithocarpus edulis. The carbon source of the denitrifiers' growth was indicated by the content of acid detergent soluble organic matter in the chips...

An in situ field demonstration was performed in fractured rock impacted with trichloroethene (TCE) and cis-1,2-dichloroethene (DCE) to assess the impacts of contaminant rebound after removing dissolved contaminants within hydraulically conductive fractures. Using a bedrock well pair spaced 2.4m apart, TCE and DCE were first flushed with water to create a decrease in dissolved contaminant concentrations. While hydraulically isolating the well pair from upgradient contaminant impacts, contaminant rebound then was observed between the well pair over 151days...

Mercury is a contaminant of global concern due to its harmful effects on human health and for the detrimental consequences of its release in the environment. Sources of liquid elemental mercury are usually anthropogenic, such as chlor-alkali plants. To date insight into the infiltration behaviour of liquid elemental mercury in the subsurface is lacking, although this is critical for assessing both characterization and remediation approaches for mercury DNAPL contaminated sites. Therefore, in this study the infiltration behaviour of elemental mercury in fully and partially water saturated systems was investigated using column experiments...

Understanding of pore-scale physics for multiphase flow in porous media is essential for accurate description of various flow phenomena. In particular, capillarity and wettability strongly influence capillary pressure-saturation and relative permeability relationships. Wettability is quantified by the contact angle of the fluid-fluid interface at the pore walls. In this work we focus on the non-trivial interface equilibria in presence of non-neutral wetting and complex geometries. We quantify the accuracy of a volume-of-fluid (VOF) formulation, implemented in a popular open-source computational fluid dynamics code, compared with a new formulation of a level set (LS) method, specifically developed for quasi-static capillarity-dominated displacement...

Enhanced upward mobility of a non aqueous phase liquid (NAPL) present in wet sand during natural drying, and in the absence of any external pressure gradients, is reported for the first time. This mobility was significantly higher than that expected from capillary rise. Experiments were performed in a glass column with a small layer of NAPL-saturated sand trapped between two layers of water-saturated sand. Drying of the wet sand was induced by flow of air across the top surface of the wet sand. The upward movement of the NAPL, in the direction of water transport, commenced when the drying effect reached the location of the NAPL and continued as long as there was significant water evaporation in the vicinity of NAPL, indicating a clear correlation between the NAPL rise and water evaporation...

A coupled experimental-modelling approach was developed to evaluate the effects of molecular weight (MW) of dissolved organic matter (DOM) on its transport through intact Boom Clay (BC) samples. Natural DOM was sampled in-situ in the BC layer. Transport was investigated with percolation experiments on 1.5cm BC samples by measuring the outflow MW distribution (MWD) by size exclusion chromatography (SEC). A one-dimensional reactive transport model was developed to account for retardation, diffusion and entrapment (attachment and/or straining) of DOM...

The use of ethanol in alternative fuels has led to contamination of groundwater with high concentrations of this easily biodegradable organic compound. Previous laboratory and field studies have shown vigorous biodegradation of ethanol plumes, with prevalence of reducing conditions and methanogenesis. The objective of this study was to further our understanding of the dynamic biogeochemistry processes, especially dissolved gas production, that may occur in developing and aging plume cores at sites with ethanol or other organic contamination of groundwater...

It is evident based on historical data that groundwater contaminant plumes persist at many sites, requiring costly long-term management. High-resolution site-characterization methods are needed to support accurate risk assessments and to select, design, and operate effective remediation operations. Most subsurface characterization methods are generally limited in their ability to provide unambiguous, real-time delineation of specific processes affecting mass-transfer, transformation, and mass removal, and accurate estimation of associated rates...

An understanding of nano-scale bubble (NB) transport in porous media is important for potential application of NBs in soil/groundwater remediation. It is expected that the solution chemistry of NB water highly influences the surface characteristics of NBs and porous media and the interaction between them, thus affecting the stability and transport characteristics of NB. In this study, in addition to stability experiments, one-dimensional column transport experiments using glass beads were conducted to investigate the effects of pH on the NB transport behavior...